1. Field of the Invention
This invention relates to the application of coating to a traveling paper web. More specifically, this invention relates to a method and apparatus for the high speed application of a film of coating material to the paper web utilizing a looped circulation path having a web interface which moves in the direction of the traveling paper web. Still more particularly, this invention relates to a method and apparatus for the application of a film of either relatively low or high viscosity coating materials, such as sizing and pigmented coating, respectively, at relatively high speeds to a traveling paper web.
2. Description of the Prior Art
One of the major problems with a conventional metering size press, which applies an aqueous slurry of sizing in an almost water-like consistency, as well as a conventional coater, which applies an aqueous slurry of pigmented coating material, which can be quite viscous in its consistency, both applied as a film to a traveling paper web, is the occurrence of two phenomena known in the papermaking industry as skip-coating and non-uniform streaks on the paper web, particularly at high machine speeds. In skip-coating, the film of coating material is intermittently interrupted in the direction of paper travel, while in non-uniform coating, machine-direction streaks of coating on the paper web have different thicknesses in the cross-machine direction. Both phenomena adversely affect the coating operation, and in many cases result in an unacceptable coated paper product.
In general, the problems relating to the non-uniform application of coating material to a traveling paper web at relatively high speeds have their basis in the failure in preventing air from entering the coating process and unfavorable vortex development in the coater head as the machine speed increases. Thus, while it is relatively easy to apply coating uniformly to a paper web traveling at 800 m/minute, for example, when machine speeds increase to relatively high speeds, such as about 1800 m/minute, for example, the dynamics of the coating process, such as the frictional interface of the traveling paper web, in a coater, or of the backing roll surface, for size press configuration, with ambient air, as well as small under- and over-atmospheric pressures created by the dynamics of the flowing coating material, induce entrained air within the aqueous slurry of coating material, which promotes the aforementioned problems.
In existing coating processes, particularly for the application of a film of relatively low viscosity, water-like sizing to printing grade types of paper, the aqueous slurry of coating material supplied to the channel or chamber exposed to the surface of the traveling paper web on a coater, or the backing roll surface, in the case of a metering size press, either flows against the paper web and out of the coater head at the downstream end, or is divided into two portions, with one portion flowing downstream and the other portion flowing upstream over a baffle and out of the coater head.
The problem with these arrangements is that neither of them works well at relatively high speeds, such as about 1800 m/minute, or greater. In the first case, because of the development of the unfavorable vortex, air is readily brought into the coating chamber by its frictional engagement with the surface of the traveling paper web (coater) or the backing roll for a metering size press. In the second case, due to the division of the flow of the coating material into upstream-directed and downstream-directed portions, in order to prevent entrainment of air traveling with the paper web or backing roll surface into the coating chamber, the flow of the portion of coating material upstream over the baffle must be sufficiently great such that there is then either an insufficient supply is coating material in the portion supplied to the coating chamber to properly coat the paper web traveling at the relatively high speed, or there is sufficient coating material to coat the paper web, but such coating is deleteriously effected due to insufficient coating in the portion flowing upstream over the baffle wall to prevent air from entering the coating chamber to be entrained in the coating material.
Some of these problems can be mitigated by either utilizing a more powerful pump, or a pump with a higher capacity for supply more coating material to the coater head. However, paper manufacturers are reluctant to incur the extra capital and operating costs associated with these alternatives. They prefer to operate with a minimum amount of coating material.
Other known prior art is shown and described in U.S. Pat. Nos. 4,834,018; 4,920,913: 5,173,120 and 5,192,591; and EP-A-0 514 735.
These documents all relate to either methods or apparatus, or both, regarding short dwell types of coater. This is the same type of coater to which this invention pertains.
In the US '018 document, there is recirculation of the coating material as well as combining the recirculated coating material with fresh coating material entering the apparatus.
In the US '913 document, there also is shown recirculation of coating material and mixture with fresh coating material.
In the EP '735 document, the fresh coating material is shown being split into upstream and downstream directed components.
In the US '120 document, a portion of the coating material is shown being directed upstream against the movement of the paper web and backing roll, and a portion of a coating material is shown being redirected back into the in-coming fresh supply of coating material.
In the US '591 document, coating material is shown being split into two portions, one portion being directed against the paper web to be coated, and the other portion being directed out of the coater head.
None of these documents show or describe a mixing chamber used in conjunction with a flow of recirculated coating material directed into the stream of fresh coating material at an acute angle.